I was a noobie computer programmer in my first job. After being there for a few months my mouse started to have issues. It stuttered and refused to move correctly.

So I'd take the little grey ball out and clean it and it'd be OK again. But the next day it'd do it again.

So I ended up with the cleanest fucking mouse on planet earth. Yet every so often it'd stop working... on parts of the screen. Other parts it'd work. I ended up using the keyboard a lot.

The mouse would work fine all day some days. Some days it would work fine until about 2PM then it may or may not work.

Eventually I figured out that when the sun shone through the window it created a beam across my mouse mat. When the mouse was moved into the beam it would stop working. So I took it to bits and discovered that the ball would roll a little disc, that disc had little holes in it, there was also an LED and a photodetector. So the mouse was counting the blips in light to determine the movement in the mouse. I guess the cheap plastic was allowing the bright sunlight to bleed through the case causing the photodetctor to go bonkers.

A secondary thing about that cheap as shit PC was you could hear the video card render the screen. It'd buzz when the screen was mostly blue, when it was white it would be silent.